None of the authentication methods supported by this client are supported by your server

In outlook, if you have customers getting the error message in the subject line, there are a couple of fixes.

Have your customers enable SSL in their email client

Disable the requirement of SSL in the WHM

The ideal solution would be to enable SSL or STARTTLS in their email client. This will make it so that when they are authenticating, it will send the login details in an encrypted form instead of plain text.

The other option would be to disable the SSL authentication requirement via the WHM. This is not recommend, as it will allow customers to authenticate using plain text, which is insecure. However, if you wish to disable it, use the following

Log into the WHM

Click on Exim Configuration Manager

Basic Editor

look for “Require clients to connect with SSL or issue the STARTTLS command before they are allowed to authenticate with the server”